Temperature control device for spark plugs



Jan. 27, 1959 e. E. MICHAUD 2,870,760

TEMPERATURE CONTROL DEVICE FOR SPARK PLUGS Filed Feb. 21, 1958 I6 26 86 Lg INVENTOR. GEORGE E. MICHAUD ATTO R NEYS United States Patent TEMPERATURE CONTROL DEVICE FOR SPARK PLUGS George E. Michaud, Salem, Mass.

Application February 21, 1958, Serial No. 716,596

5 Claims. (Cl. 123-169) This invention relates to spark plugs and in particular to a device by means of which a spark plug may be operated efiiciently under varying temperature conditions.

One of the most perplexing problems that has faced automotive engineers over the years is that of designing a spark plug for internal combustion engines that will operate efficiently over the wide range of engine speeds and temperatures normally to be encountered in use. This problem has become particularly more pressing since high octane fuels and so-called high compression engines have come into general use in automobiles.

The problem stems from the fact that an engine operated at high speeds and temperatures, such as in an automobile on'an open highway, requires what is known as a cold plug" to operate efficiently. Such a plug is necessary. to prevent pre-ignition of the fuel and accumulation of high temperature deposits. In a slower, cooler operating engine, as is found in city traflic, a hot plug is needed to insure that carbon deposits are burned oif, otherwise the accumulationof these deposits will cause fouling of the plugs with resulting misfiring. Both of these ills produce inefiicient engine operation and short spark plug life. 1 1

There is no particular problem involved in designing a spark plug for one particular engine conditionybut such a plug is efficient only for the one condition. The crux of the problem is that a hot plug will not operate satisfactorily at high engine speeds and temperatures, and likewise a cold plug will function poorly at low engine speeds and temperatures. In other words, prior to this invention there was no satisfactory spark plug available for the various driving conditions to which the average car is subjected. i

The primary object of this invention is to increase the etficiency of internal combustion engines.

Another object of the invention is to eliminate the disparity between the efiiciency of operation of aspark plug at high and low temperatures.

Still another object of the invention is to provide means for governing and controlling the operating temperature of a spark plug.

To these ends an important feature of the invention resides in providing a spark plug with an efiicient heat' conducting system automatically brought into play when the spark plug temperature reaches a predetermined value. I

Another feature of the invention resides in a heat conductive member disposed inthermally conductive relation to the shell of the spark plug and so disposed with relation to the water-cooled walls of the cylinder head as to expand and contract in and out of heat conductive contact with those walls. I

I am aware that various means have beendevised to cool spark plugsas well as means to pre-set the operating temperature of the plug. -But I am cognizant of'no arrangement, as is now provided, whereby the temperature of the plug is automatically compensated for according to the operating temperature of the engine.

In the practice of my invention, 1 surround the base shell of a spark plug with a tightly fitting sleeve, or bushing, of a metal which is not only an eflicient heat conductor but which also has a relatively high coefficient of expansion, and I provide it with a series of parallel annular fins or serrations forming circular lands about its outer periphery. The adjacent walls of the spark plug socket in the cylinder head are bored to a taper to provide a predetermined and varying clearance between the outer periphery of the sleeve and the walls of the socket, the clearance being so arranged that as the sleeve becomes heated by conduction from the shell of the spark plug, the sleeve will expand causing its fins to come progressively into contact with the surrounding walls of the cylinder head socket throughout an area which increases in extent and pressure as the sleeve becomes hotter and larger.

These and other objects and features, along with incident advantages, will be more readily understood and appreciated from a readingof the following detailed description of a preferred embodiment of the invention, selected for purposes of illustration and shown in the accompanying drawing in which the single figure is a view in cross-section through a portion of a cylinder head and one spark plug of an automobile engine, constructed in accordance with this invention. f

A cylinder head 10 of generally conventional shape is provided with an internal cooling jacket 12 and aspark plug well 14. A tapped hole 16 extends downwardly from the bottom of the well to receive the threaded base 26 of a spark plug 20. The spark plug. 20 has a conventional procelain insulator 21 surmounted by a binding post 23, a pair of depending electrodes 22, a slightly tapered shell 24 immediately below its wrench flats 25, and an annulus 86 below the shell to provide a seating surface. The shape of the well 14 departs from normal in that its walls are machined to form a chamber or socket 18 converging inwardly to a flat bottom 37 through which the hole for the spark plug is bored. The bottom of the socket thus presents an annular shoulder upon which the spark plug is seated.

When the spark plug 20 is screwed into the tapped hole 16, the periphery of its metal shell 24 faces the annular wall of the socket 18. The electrodes 22 depend 28 is also constructed with serrations or a series of.

parallel fins formed about its periphery and depicted in the drawing as having circular lands 34 and grooves 36, the lowermost land of this series being narrower than the rest. The sleeve 28 is also formed with an inwardly turned shoulder 30 of the same depth or annulus 86 to position the sleeve downwardly upon the bottom of the socket. The lower end of the shoulder 30 is bevelled to a thin edge in order that the sleeve 28 is placed only in line contact with the bottom 37 of the socket. 18.

To perform its functions which will be described and explained in the following paragraphs, it is necessary that the sleeve 28 be made of a material having a greater coefficient of expansion than the walls of the chamber in the cylinder head which is customarily of cast iron. It is also necessary that the sleeve have a coefiicient of 3 thermal conductivity at least as high as the cylinder head. To these ends aluminum is particularlywell suited although the invention is not intended to beso limited, it being understood that other metals may be substituted.

The embodiment of my invention operates on the following principles. The flow of heat from the high temperature region of a spark plug is governed by (a) the thermal conductivities and geometric distribution of the porcelain, electrode, and steel body of which the spark plug is composed and (b) by the area and pressure of total physical contact between the spark plug and the surrounding water jacket. At a given firing and fuel rate, the greater the physical contact between the spark plug and the water jacket, the lower is the steady operating temperature of the porcelain in the hot region of the spark plug. At low firing and fuel rates it is desirable that the heat flow path between the porcelain and the water jacket 'be sufiiciently limited that the porcelain temperature may become high enough to avoid formation of low temperature deposits; at high fuel and firing rates the same degree of thermal contact will inadequately cool the porcelain, and high temperature deposits will form. Thus if a spark plug has a fixed degree of thermal contact with the cooled cylinder head, and is designed to function perfectly at intermediate speeds, at low speeds it will. operate too cold and at high speeds too hot with the consequence that, in variable speed operation, deposits will accumulate which can eventually destroy the dielectric characteristic of the exposed porcelain surface.

In a conventional spark plug, which is in constant thermal contact with the water cooled head, the rate with which heat flows from the spark plug to its environment is very nearly directly proportional to the temperature difference between the plug and the head. This invention relates to a device which increases the thermal contact'between the plug and the cylinder head as the temperature of the plug increases, so that an increase in plug temperature will result in a disproportionately large increase in heat flow rate, as will be shown below. Similarly a decrease in plug temperature will effect a disproportionately large decrease in heat flow rate. The object and consequence of establishing variable contact between the plug and the head is to cause the plug temperature to remain within a relatively narrow range even with wide variations in engine speed; in this way the formation of either high or low temperature fouling deposits may be minimized.

In a conventional plug, during operation, heat flows through the electrode and porcelain in an axial direction, parallel to the electrode, away from the firing end of the plug, and radially through the porcelain to the steel jacket of the plug. Heat leaves the steel jacket (a) by conduction through the spark plug gasket or seat into the cylinder head, and (b) by convection from the steel jacket into the surrounding air. Since the electrode has a thermal conductivity approximately 30 times that of porcelain axial heat How is mostly along the electrode. Radial heat flow from the electrode to the surroundings is primarily limited by the convection of heat from the steel jacket to the air, and is secondarily limited by radial conduction through the porcelain. There is relatively little resistance to the flow of heat through the metal itself, and consequently the temperature gradient across the thickness of the steel jacket is quite small. Thus the operating temperature of the hot end of the plug can be reduced by augmenting heat transfer from the steel jacket to the cylinder head, an effect best accomplished by increasing the metal-to-metal contact between the jacket and the head over and above that provided by the spark plug gasket or seat. By using an expandable metal insert the degree of metal-to-metal contact may be made dependent on temperature, with the result that the operating temperature of the plug will be nearly stabilized.

In this invention, the expandable metal insert is the slightly tapered, serrated or finned sleeve 28, which is of a metal having high thermal conductivity and preferably a coefficient of thermal expansion greater than that of steel, and also possessing some characteristics of elasticity so that the insert does not become deformed from the heat and pressure to which it is subject in operation. The smooth inner surface of the insert 28 and the exterior of the steel jacket 24 on the plug are machined to a common taper, so that the insert will have a sliding fit (at room temperatures) over the exterior of the plug. The serrated exterior surface of the insert nearly mates with the spark plug well, which now must be a machined tapered socket 18 in the cylinder head instead of the usual cast-in well. The clearance 32, between the serrated exterior of the insert and the machined surface of the well increases with the distance from the hot end of the plug, so that when the insert is heated and expands, the lower lands of the serrations bear against the well first, and, as the temperature increases further, the lands progressively come to bear until all are in metal-to-metal contact with the head.

With the insert designed so that nearly all the heat transmitted to the insert comes from the steel spark plug jacket, and the insert has a higher coeflicient of thermal expansion than steel, (a) contact will be maintained between the steel and the insert, (b) the steel will be predictably hotter than the insert, (c) the rate of heat transfer from the steel to the insert will be roughly proportional to the pressure of contact between the steel jacket and the insert, (d) the rate of heat transfer from the steel jacket to the insert will equal the rate with which heat is transferred from the insert to the surroundings (mainly to the cylinder head 10) and (e) the rate of heat transfer from the insert to the water jacket 12 is governed by the number of serrations on the insert which bear against the socket 18, and by the pressure prevailing on these bearing surfaces.

As long as heat is being supplied to the insert more rapidly than heat can be transferred from the insert to the head, the insert will increase in temperature and expand, reducing the pressure between the insert and the plug, until a steady condition is attained. Conversely, since the insert is always losing heat to the surroundings, it will always contract enough to maintain some degree of thermal contact with the plug, from which the heat must originate. This follows from the fact that should the insert start to expand away from contact with the plug, the source of heat flow to the insert would diminish accordingly with the result that the insert would cool and contract into thermal contact with the plug.

; It has been determined that the contact between the sleeve 28 and the bottom 37 of the socket 18 must be limited, since a large contact area in that location merely operates to produce a cold plug. Hence the bottom of the shoulder 30 is reduced to a thin edge in order to limit the contact area to line contact. Alternatively the bottom of the sleeve 28 may be provided with three or more small buttons in order more definitely to limit the contact. Y

Since thermal conditions dictate that light or heavy contact be maintained between the plug and the insert,

and that temperature gradients across the metal members themselves are quite small, if the inner surface of the insert 28 and the outer surface of the steel jacket 24 are coincident at room temperature T and nearly so at elevated temperatures, the temperatures of the insert, T depends on the temperature of the steel jacket, T

where X =coeflicient of thermal expansion of steel F."' X =coefiicient of thermal expansion of insert.

The rate of heat transfer from the steel jacket to the insert is given by:

where,

q=rate of heat transfer, B. t. u./hr.

h =coeflicient of heat transfer between steel and insert,

B. t. u./ft. hr. F.

A =contact area between steel and insert, ft.

The coeflicient of heat transfer, H increases with pressure, and may vary from 100 B. t. u./ft. hr. F. with very light contact to over 5000 when the interface is under high pressure.

The rate of heat transfer from the insert to the water jacket is dependent upon the number, n, of rings or serrations which bear on the machined well, and must be steady operation equal the value of q given by Equation 2 where,

h =coeflicient of heat transfer between a ring serration and the well,

A =contact area of a single ring serration with the well T =temperature of the cylinder head (well)..

Consider, for example, that the insert be designed so that two (of seven) rings are engaged when T,; is 300 F., and that six are engaged when T =500 F.

When T =300 F., from Equation 1:

If the rings are under moderate pressure; h; might equal about 1000, and, at the low operating temperature, T might reasonably be taken as 120 F., from Equation 3:

=13O B. t. u./hr.

From Equation 2:

In other words, very light contact between the plug and the insert would be sufficient to maintain the low rate of heat transfer.

When T :500" F., from Equation 1 T =140 F., from Equation 3;

=1000 (6) 0.001 (285-140)=870 B. t. u./hr.

From Equation 2:

q=870=h (0.01)(500-285) h =405 B. t. u./ft. hr. F.

The relatively low value of k means that light contact is still sufficient to remove almost seven times as much heat at 500 F. as was removed at 300 F. The calcula- ,tion indicates the feasibility of amplifying heat removal greatly with but slight increases in plug temperature. Further increases in temperature would not only bring another ring to bear, but would increase the coeflicient by augmenting pressures.

Typical design clearances can be calculated from the above values, at the level of the second ring, if the outer diameter of the ring is taken as 0.716 inch, and is supposed to come into contact when T =300 F. and T,-=185 F., the ring should have a dimetral room temperature clearance of:

0.716 X,- (l70)=0.001 inch At the level of the sixth ring, if the outer diameter. is taken as 0.741 inch, and is supposed to come into contact when T =500 F. and T =285 F., the clearance should be:

0.741 X, (285- 70)=0.002 inch A reading of the foregoing description will suggest numerous minor variations to those skilled in the art to which this invention pertains. For example, although the description generally has been limited to automobile engines, with slight modifications the concept may obviously be employed successfully in most other forms of internal combustion engines. Therefore, it is not intended that the foregoing description limit the breadth of this invention, but that the scope be defined by the appended claims and their equivalents.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an internal combustion engine, a spark plug having a metal shell, walls defining a socket for the spark plug, and a metal externally serrated sleeve engaging said metal shell, the sleeve and walls being dimensioned to provide between them an annular space when unheated, said sleeve having a greater coefficient of expansion than that of the walls of the socket, whereby when the engine is heated during operation the sleeve will expand to meet the socket walls, the contact area and contact pressure increasing with the temperature.

2. In an internal combustion engine, a spark plug having a metal shell, a metal cylinder head having walls forming a socket for said plug, a sleeve of metal having greater coefiicient of expansion than that of the metal of the cylinder head and being annularly serrated in lands about its external surface, said sleeve fitting upon said shell and forming with the socket an annular chamber having outwardly diverging walls, whereby when the engine is heated during operation, the sleeve will expand the lands of the serrations into progressive contact with the socket wall, the contact area and the contact pressure increasing with temperature.

3. In an internal combustion engine, a spark plug having a tapered metal shell, walls defining a tapered socket for the spark plug, and a sleeve provided with external annular serrations, said sleeve engaging with said metal shell, said sleeve having an inwardly turned bevelled shoulder formed at the base of said sleeve, the sleeves and walls being dimensioned to provide between them an annular space increasing in volume upwardly when unheated, said sleeve having a greater coefficient of expansion than that of the walls of the socket, whereby when the engine is heated during operation the sleeve will expand to meet the socket walls, the contact area and contact pressure increasing with the temperature.

4. In an internal combustion engine having a cylinder head with a spark plug socket presenting an annular shoulder at its lower end and upwardly diverging walls, a spark plug in said socket having a tapered metal shell spaced from the walls of the socket, and a metal sleeve engaging the shell of the spark plug, diverging upwardly from the walls of the socket and having a bottom flange shaped to make line contact with the annular shoulder of the socket.

7 8 5. In an internal combustion engine having a cylinder as said annulus and beveled to make line contact with head with a spark plug socket presenting an annular the annular shoulder of said socket.

shoulder at its lower end and upwardly diverging walls,

a spark plug in said socket having a tapered metal shell References cued m the file of thls patent spaced from the walls of the socket, said shell having 5 UNITED STATES PATENTS an annulus at its base, providing a seating surface for 1,564,950 Curran Dec. 8, 1925 said spark plug on said cylinder head, a tapered thermo- 1,636,197 Rohde July 19, 1927 static sleeve, externally serrated, engaging the shell of 2,055,204 McKane Sept. 22, 1936 the spark plug, diverging upwardly from the walls of 76,038 MCKane Apr. 6, 1937 the socket and having a bottom flange of the same depth 10 2,212,725 AndfeS A g- 27, 1940 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0. 2,870,760 January 27, 1959 George E. Michaud It is herebfi certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 5, line 17, for "must be read must at line 30, for

"degree or" read degree of Signed and sealed this 12th day of May 1959.

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner of Patents 

